Stressed out moms mean faster growing babies—if you’re a squirrel

For the offspring, it's grow fast, die young.

To give her offspring better odds in life, a mom can contribute all sorts of advantages: good genes, healthy milk, protection from predators, and more. Red squirrels—cousins of the more robust gray squirrels—are no exception. But new research in the journal Science shows that for these rodents, one such maternal boost stems from a surprising source: stress.

The research was conducted by a group of scientists who have spent the last 22 years studying a group of wild red squirrels in the Yukon. The ecosystem there follows an episodic pattern: every few years, when the spruce trees produce seeds en masse, the squirrel population booms in response. The team’s previous research had shown that in the years when squirrel numbers are especially high, fast-growing offspring tend to fare better than those that grow more slowly, presumably since they are better equipped to outcompete their rivals.

But how, exactly, do some squirrels end up growing so quickly?

Since female squirrels feed and care for their young, the researchers focused on how they responded to a booming squirrel population. They broadcast squirrel vocalizations over loudspeakers, signaling to wild squirrels that the environment was getting crowded. Once the female squirrels gave birth that season, the researchers monitored the offspring’s development by weighing the pups just after birth and again about 25 days later.

The offspring of female squirrels that had been exposed to the loudspeakers grew much faster than the offspring of control squirrels, which didn’t hear any extra squirrel vocalizations. In other words, a mom hearing extra squirrel calls before her pups were born somehow sped up their development.

Increases in growth rates are often attributed to concurrent increases in food availability, but that wasn’t the case here—this effect was seen even without the extra food resources that often accompany times of high population density. Instead, a mere social cue appeared to trigger the boost in the pups’ growth rate.

The key, the researchers realized, was that the females’ stress hormones—called glucocorticoids—were rising, thanks to the perceived overcrowding. A separate experiment, in which the scientists tested the effect of crowded conditions on stress levels, showed a positive relationship between population density and glucocorticoid levels: the more crowded the environment, the more stressed pregnant squirrels became.

And this increase in stress levels, according to the researchers, can affect pups’ development. To confirm this link, they experimentally boosted the stress levels of pregnant female squirrels by feeding them cortisol. The offspring of these stressed-out females grew 41 percent faster than the offspring of control females that hadn’t been fed the hormone, confirming that a mom’s stress—specifically, her glucocorticoid levels—can vastly change how fast her pups develop.

Stress is often considered to be a bad thing; we constantly hear about the health consequences of stress, and doctors often advise pregnant women to avoid stressful situations. But for red squirrels, stress brought on by social cues may be important to help offspring start off on the right foot. Stress plays an important role in many animals' lives, spurring them to seek out new territory or run from predators, among other things. But until now, there has been little evidence that a mother's stress can actually benefit her offspring.

There is no evidence that this stress harms the moms in this scenario; female squirrels that gave birth to fast-growing young didn’t appear to suffer any survival costs. However, there do appear to be ramifications for the pups. Fast-growing squirrels tend to die earlier than their slow-growing counterparts, suggesting that there are consequences for this advantage early in life.

Researchers don't yet know exactly how glucocorticoid levels in utero affect growth rate or why fast-growing pups don’t live as long. What is clear, however, is that stress may have a more intricate relationship with fitness than scientists previously thought.

Kate Shaw Yoshida
Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas. Emailkate.shaw@arstechnica.com//Twitter@KateYoshida

"But for red squirrels, stress brought on by social cues may be important to help offspring start off on the right foot."

I don't think this is quite right. I would say that the mother's stress is being used to help improve the ability of the offspring to survive in an environment with that level of stress.

It's not stated here, but growing faster than normal almost certainly is carrying some kind of cost. Might be an immune system cost, lifespan cost, etc. A reason why squirrels don't normally grow up at the accelerated rate.

So the response to stress is not helping the squirrels in absolute terms, it is helping them prepare for a particular set of environmental conditions. They would still probably be better off without the stress at all, allowing them to grow at a normal pace. But if the stress is present, then they may be better off growing fast to deal with it.

I suspect that this effect is more widespread than implied here, and growth and development is commonly affected by environmental factors. I wonder which of those factors is at play in human growth and development. What kinds of trade-offs do we make to deal with sub-optimal conditions?

Just about any sounds blasted by loudspeakers increase stress. I think the researchers are reaching when they ascribe specific reasons such as (faked) squirrel overpopulation to the stress the mothers experienced.

I cannot help but wonder if the process described in the article is less about the Mother Squirrel giving her offspring a chance and more about saving herself. Motherhood for those species that raise their young is a significant investment in time and energy. Perhaps she is working to get her progeny out of the house, so to speak, so that she may survive to breed again in less difficult circumstances.

Stress in rodents caused by auditory stimulation has previously been noted. I've cited this article before on Ars, but it bears repeating - Music can be hazardous to mouse health. (Warning: Probably not serious science)

Just about any sounds blasted by loudspeakers increase stress. I think the researchers are reaching when they ascribe specific reasons such as (faked) squirrel overpopulation to the stress the mothers experienced.

To clarify: the control squirrels also were exposed to playbacks, but rather than squirrel vocalizations, they heard bird calls.

Since female squirrels feed and care for their young, the researchers focused on how they responded to a booming squirrel population. They broadcast squirrel vocalizations over loudspeakers, signaling to wild squirrels that the environment was getting crowded.

Until I figured out the intended antecedant, I imagined the squirrel equivalent of an 80's era MC.

Stress is often considered to be a bad thing; we constantly hear about the health consequences of stress, and doctors often advise pregnant women to avoid stressful situations. But for red squirrels, stress brought on by social cues may be important to help offspring start off on the right foot.

This is presented as an apparent contradiction, but it's really not. Human health-and-wellness types care about lifespan and quality of life - the goal is to maximize the period of time that the organism is able to function independently and with sufficient capacity; it's a very complex topic, obviously. Evolution cares about EXACTLY one thing: how many offspring does the organism have, and how healthy are they (i.e. how likely are they to reproduce). From an evolutionary point of view, it's entirely reasonable to burn out the parental organism to maximize the offsprings' chances; and once the organism is past reproductive age, evolution cares not much about it's fate. Very, very different sets of priorities.